Modern Vespa

My 2005 4T LX50 seems to be getting poor mileage, so I thought I would check the idle adjustment and air/fuel mixture adjustment. The guides I have read state that the scooter should start to falter as you turn the air/fuel mixture screw in (lean the mixture out), however, my LX50 idles fastest when the screw is fully in, and starts to falter when it is about two turns out. This suggests that it isn't getting enough air (or too much fuel even when fully lean). I'm not sure where to go from here. Any advice is appreciated!

Have you check your jets for cleanliness?

I haven't, but I will do. Could dirty jets cause it to run rich?

Check your intake manifold for leaks/cracks also.